Such electric vehicles may be driven by a purely electrical power unit, or they may be driven by a mixed power unit, which consists of an electric motor together with an auxiliary heat engine. The heat engine can either drive a generator for recharging the power batteries (in the case of so-called hybrid vehicles), or it may also serve as a propulsion unit for direct propulsion of the vehicle (in the case of "dual model" vehicles). Such mixed propulsion vehicles are also referred to as "lean and clean" vehicles, due to the fact that the auxiliary heat engine is not intended to be used in urban situations.
The electric propulsion motors of electric vehicles give off only a small amount of heat, generally in an amount of the order of a few hundred watts, as compared with the heat engines of conventional motor vehicles which emit very much more heat.
In conventional motor vehicles, the high heat loss of the engine is used for the purpose of heating the cabin of the vehicle. To this end, a heating radiator is supplied with engine cooling fluid, and an airstream is passed through the radiator and thence delivered into the cabin. By contrast, in an electric vehicle, whether having purely electrical propulsion or mixed propulsion, the heat output emitted by the propulsion unit has up to the present time been regarded as much too small to be useful for heating the cabin. This is why heating of electric vehicles is usually obtained by means of an electric heating radiator or by a fuel burner.
The main disadvantage of an electrical heating radiator is that it is supplied with power from the on-board source of electrical energy, which is detrimental to the autonomy, or independence, of this type of vehicle. Such autonomy is already severely reduced by the limited capacity of the propulsion batteries. As to the use of a fuel burner, the disadvantage of this is that two different energy sources have to be provided on the vehicle, and such a burner is of course also a source of pollution.